Organic solvent permeation through HKUST-1 MOF membranes: Molecular-scale insights

Abstract

HKUST-1 is a popular metal-organic framework (MOF) that is widely for organic solvent nanofiltration (OSN). This study targets a molecular-level understanding of the organic solvent permeation characteristics of HKUST-1-based OSN membranes. Non-equilibrium molecular dynamics (NEMD) simulations are conducted to investigate the permeation of two polar (namely, methanol and acetone) and five non-polar (namely, hexane, cyclohexane, toluene, xylene, and CCl4) organic solvents through the HKUST-1 membrane. Compared to polar protic methanol and polar aprotic acetone, non-polar solvents exhibited higher solvent flux due to their weaker interactions with the HKUST-1 membrane, except for CCl4, which has the highest viscosity and relatively larger molecular diameter. Among the non-polar solvents, the highest permeation flux exhibited by hexane (256 x 104 +/- 24 x 104 kg m-2 h-1) is attributable to the lowest viscosity, narrow potential mean force distribution, relatively lower energy barrier, and loss of long-range interactions. Furthermore, for slow-permeating solvents, the moving wall NEMD (MW-NEMD) approach is recommended, due to its straightforward implementation and ease of use. On the other hand, for fast-permeating solvents, the boundary-driven NEMD (BD-NEMD) is more effective, as it allows for extended simulation times with reasonably sized systems, avoiding the need for large-scale setups and thereby reducing computational costs. The molecular-level insights revealed here are expected to be valuable in the advancement of MOF membranes for OSN.